1. Field of the Invention
This invention relates to a shape memory alloy (SMA) suitable for actuators and a method of treating the same.
2. Related Art
Heretofore, upon treating a raw shape memory alloy so as to make it suitable for use in actuators, generally it has not been done to refine crystal grains and control crystal orientations of the raw shape memory alloy.
On the other hand, in order to use a shape memory alloy, it is necessary to impart a required shape to the shape memory alloy, and therefore to perform a heat treatment peculiar to each kind of shape memory alloy. This heat treatment is called shape memory treatment and it is necessary to strictly control various conditions thereof, as it is a very delicate treatment. For example, the following methods have been well known as shape memory treatments for common Ti—Ni based shape memory alloys. The first method, which is referred as medium temperature treatment, is the one wherein a shape memory alloy is sufficiently work hardened and then cold worked into a desired shape, and thereafter, held at a temperature of 400 to 500° C. for a few minutes to several hours with the desired shape being restrained. The second method, which is referred as low temperature treatment, is the one wherein a shape memory alloy is held at a temperature of 800° C. or above for some time, thereafter rapidly cooled and cold worked into a desired shape, and then held at a low temperature of 200 to 300° C. with the desired shape being restrained (Illustrated idea collection of applications of shape memory alloys in the latest patents, written and edited by Shoji Ishikawa, Sadao Kinashi and Manabu Miwa, published by Kogyo-chousa-kai, pp. 30).
In general, conventional shape memory alloys suffer from the following shortcomings when used in actuators.
(a) The response characteristic (speed) is inferior.
(b) Usable temperature range is restricted, since Ms and Mf points (Ms being the temperature at which the martensite phase transformation starts and Mf being the temperature at which the martensite phase transformation ends) are difficult to be raised.
(c) Only a small force can be effectively extracted from the shape memory alloy.
(d) The service life before being broken is short.
(e) The shape memory alloy tends to lose the memory of an imparted configuration and permanent strain tends to be produced in the shape memory alloy for a short period of time.
(f) The strain which can be extracted from the shape memory alloy as a movement (hereinafter referred as operational strain) is decreased for a short period of time.
(g) Shape memory alloy materials, such as Ti—Ni based or Ti—Ni—Cu based alloys and the like, which are intermetallic compounds having strong covalent bonding characteristic and are difficult to work, are difficult to use when they are in certain compositions, since they are very brittle and fragile.
With such shortcomings, 80 to 90% or more of applications of shape memory alloys have been those wherein they are used as superelastic spring materials and only the rest has been directed to actuators. Moreover, most of the shape memory alloys for use in actuators have been formed into the shape of a coil spring, wire or plate and have been expected to be reverted from a configuration deformed by bending or twisting and bending to the original configuration upon application of heat (in case the shape memory alloy is formed into a coil spring shape, though macroscopically or apparently it is deformed as if it were elongated or compressed upon application of a force thereto, in a true sense the deformation it is subject to is a twisting and bending one). The reason for utilizing reversion from a bending deformation or twisting and bending deformation as stated above has been that the shape memory alloy should be used so that its small strains may be multiplied since the range of its shape memory effect (SME) stably available is very narrow. Though it is said that, in conventional shape memory alloys, the maximum operational strain reaches a few percent to about 10 percent, this is true only when deformation and shape recovery are performed only once or a few times. Practically speaking, when deformation and shape recovery are repeated over large cycle numbers with regard to the conventional shape memory alloy, the operational strain is decreased and the alloy loses the memory of the imparted configuration and eventually is broken.
All of the conventional shape memory treatments intend to keep the shape stability while obtaining the pseudoelasticity and shape memory effect by partly producing microstructures which can cause pseudoelasticity and shape memory effect in microstructures of the shape memory alloy strengthened by work hardening. In other words all of the conventional shape memory treatments are those which obliges to sacrifice pseudoelasticity and shape memory effect to some extent to keep shape stability.
On the other hand, the present inventor has disclosed in U.S. Pat. No. 4,919,177 a method of treating Ti—Ni based shape memory alloy wherein a Ti—Ni based polycrystalline shape memory alloy material is subjected to a heat cycle which rises and drops over the transformation region of the shape memory alloy as well as to a directional energy field. According to this method, the crystal orientations of the shape memory alloy are rearranged along a specific direction and the disadvantages of the conventional shape memory alloy are overcome considerably.
However, in the method disclosed by the present inventor, the crystal grains of the shape memory alloy are not refined but caused to grow in size. Besides, since a tensile force is applied to the shape memory alloy in the final step of arranging the crystal orientations, there is a tendency that the microstructure of the shape memory alloy finally obtained is destroyed by the tensile force. Therefore, it is still not enough in overcoming the disadvantages of the conventional shape memory alloy.